[zeromq-dev] Notification of peer disconnection for ZMQ_STREAM sockets.

Pieter Hintjens ph at imatix.com
Sat Jan 11 10:51:07 CET 2014


Hi André,

Yes, this would be a good solution. You'll have to ask someone to help
make the patch, or learn enough to make it yourself.

-Pieter

On Thu, Jan 9, 2014 at 6:05 AM, André Caron <andre.l.caron at gmail.com> wrote:
> Hi there!
>
> First and foremost, kudos for all your awesome work on this excellent
> library :-)
>
> I'm experimenting with ZMQ_STREAM sockets and I'm not sure how to handle
> disconnection of peers.  The man page is pretty clear on how to forcibly
> disconnect a peer (send 0-length message), but there is no information about
> handling disconnections.  Local tests using a simple Telnet server
> implementation (pet project to accept control commands via Telnet in
> ZMQ-based nodes) show me that the program never gets notified if the peer
> disconnects (at least, zmq_poll never marks the socket as readable).
>
> This piece is quite critical because an application that receives non-framed
> messages (such as an HTTP server) maintains per-connection state: in
> particular, you need to buffer request data until a full request has
> arrived.  This state must be dropped if the peer disconnects or the
> connection is lost.
>
> Without some means to detect that a connection is no longer usable, ZMQ
> programs must hold a per-connection timeout, and zmq_poll() using the min of
> all these timeouts and forcibly drop the state for whichever connection has
> expired, and then tell ZMQ to drop that connection.  Apart from introducing
> an unnecessary delay in cleanup, this is quite a bit of work to be repeated
> in each place where we use ZMQ_STREAM sockets... (I'm not hoping to do this
> routinely, it would still be a pain to maintain).
>
> It seems to me that ZMQ_STREAM is so close to looking like a real ZMQ
> socket.  The last thing it needs IMO to be close enough would be to have the
> following behavior on peer-initiated disconnection:
> - zmq_poll() shows disconnected ZMQ_STREAM sockets as readable; and
> - zmq_msg_recv() return as zero-length message.
> This would make for such a smoother experience of bridging ZMQ with existing
> protocols!
>
> Any thoughts on this?
>
> Cheers,
>
> André
>
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